1. Field of the Invention
The present invention relates to a semiconductor device having a charge transfer device, MOSFETs, and bipolar transistors all formed in a single semiconductor substrate.
2. Description of the Related Art
Various charge transfer devices are known, among which is a charge-coupled device (hereinafter referred to as "CCD"). The CCD is formed in a semiconductor substrate, along with N-type channel MOSFETs which constitute various peripheral circuits for the CCD, such as a circuit for switching CCD delay lines, a CCD-driving circuit, a sample-and-hold circuit, an output circuit, and a clock-driving circuit. Hitherto, a 12 V or 9 V power supply has been used to drive the N-type channel MOSFETs. Recently, a 5 V power supply has been used instead, to save power, to cope with the increasing use of CMOS including MOSFETs, and to match the power-supply voltage of the MOSFETs with that of bipolar transistor formed on a separate chip.
When driven by a voltage of 5 V, the sample-and-hold circuit and the output circuit, either formed of N-type channel MOSFETs and being an operational amplifier, fail to produce outputs having a sufficient linearity. The poorer the output linearity of these operational amplifiers, the lower the yield of the semiconductor device having the CCD and these peripheral circuits, as is usually proved by die sort test.
The CCD delay lines are designed to delay signals by a predetermined period of time. Most circuits for processing the signals thus delayed are bipolar transistor ICs which are formed on separate chips.
Let us assume that the CCD and bipolar transistors are formed in the same semiconductor substrate, thus replacing the operational amplifiers having MOSFETs, such as the sample-and-hold circuit and the output circuit, with operational amplifiers comprising the bipolar transistors which operate at higher speed than the MOSFETs. In this case, the operational amplifiers have better output linearity, and the semiconductor device having the CCD and these operational amplifier can be manufactured with higher yield and can operate with higher efficiency. In addition, the use of the bipolar transistor IC, which processes the signals output by the CCD, results in space merit, reduction of manufacturing cost, increased device function, and a simplified system.
To make the CCD transfer an electrical charge, a transfer clock signal is supplied to the CCD. Some part of this clock signal leaks and makes noise, while being supplied through the semiconductor substrate. The noise adversely influences the bipolar transistors. In view of this, it is far from advisable to form bipolar transistors together with a CCD in the same semiconductor substrate. If influenced by the noise, bipolar transistors will fail to exhibit good characteristics, and the semiconductor device comprising the CCD and the bipolar transistors will have insufficient reliability.